There are certain types of torque and force generating actuators which are intended to operate a variety of devices such as valves and dampers. Actuators are classified as either rotary or linear depending on the kind of output provided. However, with the proper type of linkage, rotary actuators may be adapted to provide linear force to a load, and linear actuators may be adapted to provide torque to a load. Rotary actuators are further subdivided into those providing torque through a fraction of a complete rotation and those which can rotate continuously in one direction. In either case these rotary actuators typically rotate quite slowly, with fractional turn actuators typically rotating at a speed of less than one RPM and continuous actuators rotating at a few RPM.
Internally, an actuator typically comprises a small electric motor driving a high ratio reduction gear train so as to reduce the output speed to a level appropriate for the operated device. The motor may drive through a magnetic torque-limiting clutch which allows the actuator to stall for long periods of time with no harm to any of its components. An arm linkage, cam, or rack and pinion can be used to convert the rotational output of the gear train to linear output when required. Actuators usually have the capability of changing drive direction so that the device which is being operated can be driven back and forth between two different positions, with the common option of stopping at any desired point between the two positions. There is usually internal friction in the gear train adequate to prevent the driven device from shifting from its current position when the actuator is inactive. Where this is not true, an internal brake may be provided to hold the driven device in the current position when the actuator is inactive.
Every actuator has some kind of output element to which the device which it drives is connected and which provides the output force or torque generated by the actuator. For rotary actuators there are two main types of actuator output elements. A hub type output element usually has a square or splined bore extending completely through the hub and into which an operated device input shaft fits. A second type of output element comprises a projecting shaft which may be square or splined or have a flat surface for a set screw fastener to which the operated device can connect. Hub type output elements have the advantage of allowing the operated device's mating shaft to be inserted from either end of the hub. Either type of output element usually relies on set screws or U bolts to securely and detachably connect the output element to the device. Square connection cross sections are often preferred over other types because they are relatively cheap to form and have high torque transmitting characteristics. Square shafts and hub bores do have the characteristic of allowing only 90.degree. increments in the orientation of the hub relative to the operated device shaft. In certain situations, say those involving fractional turn actuators, this is disadvantageous.
There is a particular application for a continuously rotating actuator which involves rotating a jackscrew to linearly position a damper or door for example. Such a jackscrew comprises a rod carrying usually Acme or square threads. A traveler with mating threads in a bore is carried on the jackscrew. Rotation of the jackscrew translates the traveler to move the operated device. Operation of the jackscrew for such an application creates a substantial axial load on the jackscrew, which most conveniently is carried by the actuator's output element itself. Jackscrews with their essentially circular cross section cannot be directly connected to the standard actuator output elements. In particular, it is not easy to connect such a jackscrew to an actuator hub having a square bore connection. This is desirable for the simple reason that these hubs are already widely used, so little additional tooling is required to adapt them for this application. Thus, a specialized connection to a square bore output element is required which can handle both torque and axial loads from the jackscrew, and which provides both radial and axial support for the end of the jackscrew attached to the output element. Preferably, this connection should easily attach the jackscrew to the output element, maintain the connection reliably, and allow easy disconnection for adjustment or repair. It is also desirable that the connection provide a small amount of flexibility between the hub and the jackscrew so that axial misalignment between the jackscrew and the hub does not create undue wear on the hub bearings or the hub drive gear.
One solution which suggests itself is to place a stub shaft in the square hole, and connect the jackscrew to the stub shaft. One tried and true arrangement for connecting two shafts involves creating a slot in one and in the other a tab or finger which fits into the slot. A hole is formed in the material defining the slot in the one shaft, and in the other's tab. A pin of some type can then be inserted through the holes to connect the two shafts. Although simple to design, this arrangement has a number of disadvantages. It is difficult to drill the hole in a shaft such as a jackscrew made of hardened steel. The diameter of the pin must of necessity be much smaller than that of either shaft, and yet must carry all of the axial load applied to the shafts. This creates a potential for premature failure of the pin either through overloading or wear which reduces the pin size to a point where failure can occur. Lastly, manufacturability is an issue. Particularly in a crowded space, trying to align both shafts so that the pin can be inserted through their holes can prove difficult and time-consuming. Then, some type of retaining feature must be applied to the pin. For example, if the pin is a machine screw, a nut must be turned onto the end of the screw, and must have some type of locking feature which prevents the nut from working loose during use. If a cotter pin is used, there are issues of strength because of its lengthwise split, and its end must be bent after insertion. But good engineering practice discourages using a cotter pin as a force or torque-carrying member.
There are other types of special actuator-operated devices which are most convenient to connect to existing hub designs. Many of these are more easily connected to shafts than directly to the connection hole in the hub. It is possible to redesign these hubs to provide some type of shaft connection structure. Experience has shown that this approach is less flexible and fails to cooperate well with existing hub structures. Accordingly, we have developed a new type of system for attaching various types of operated elements to the actuator providing the torque or linear force.
U.S. Pat. No. 892,021 (Wirsching) shows a pair of resilient arms each having an outwardly projecting bump at its free end. The arms can be pressed toward each other so as to allow both bumps to enter a rectangular hole in the center of a flat hub such as that of a pointer. Releasing the arms allows the spring force of the arms to retain the hub on the arms.